1. Field of the Invention
The present invention relates to a cathode-ray tube and, particularly, to an improvement of an electrode plate which constitutes an electron gun of a cathode-ray tube.
2. Prior Art
A cathode-ray tube (hereinafter referred to as a color cathode-ray tube) used for color image display is constituted by a panel unit which is an image screen, a neck unit which holds an electron gun, and a funnel unit which couples the panel unit to the neck unit. In the funnel unit is mounted a deflector which causes an electron beam emitted from the electron gun to scan a fluorescent screen applied to the inner surface of the panel.
The electron gun held in the neck unit is provided with various electrodes such as a cathode electrode, a control electrode, a focusing electrode and an acceleration electrode. The electron beam from the cathode electrode is modulated by a signal applied to the control electrode, and is permitted to impinge on the fluorescent screen after having been imparted with a required sectional shape and energy through the focusing electrode and the acceleration electrode. In the course of arriving at the fluorescent screen from the electron gun, the electron beam is deflected in a horizontal direction and in a vertical direction by the deflector provided in the funnel unit so as to form an image on the fluorescent screen (Japanese Patent Laid-Open No. 215640/1984).
FIG. 16A is a plan view of an electrode (G3 electrode) which constitutes the electron gun provided in a conventional cathode-ray tube, and FIG. 16B is a sectional view of the G3 electrode along the line B-B' of FIG. 16A. In these drawings, symbol G3 denotes a G3 electrode, E.sub.1 denotes a first electrode plate which constitutes the G3 electrode G3, symbol E.sub.2 denotes a second electrode plate which constitutes the G3 electrode G3, symbols H denote beam passage holes. Each of the first and second electrode plates E.sub.1, E.sub.2 has three in-line beam passage holes H. Symbols S denotes bead supports (supports of bead glass not shown) provided to the first electrode plate E.sub.1.
A conventional G3 electrode G3 has been formed by welding two electrode plates together, i.e., by welding together a first electrode plate E.sub.1 having bead supports S and a second electrode plate E.sub.2 having three beam passage holes H. Therefore, the thickness of the first electrode plate E.sub.1 where bead supports S are formed is different from that of the second electrode plate E.sub.2 where the beam passage holes H are bored, developing steps in the boundary between the two. The reason why the plates with different thicknesses are used and a step is formed is to decrease the gap between the G2 electrode (not shown) and the G3 electrode G3 in order to improve the focusing performance without deteriorating the breakdown voltage characteristics.
Conventionally, as shown in FIGS. 16A and 16B, since two electrode plates E.sub.1 and E.sub.2 are welded together, the productivity is low and the manufacturing cost is high. Furthermore, when a piece of electrode plate is subjected to coining by press-machining in order to obtain an electrode having a step, there arises a problem that the tools are often damaged due to the lack of sufficient strength.